Epoxy resins have the excellent characteristics of moisture, solvent and chemical resistance, toughness, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. The versatility in formulation also make epoxy resins widely applicable industrially for surface coatings, adhesive, painting materials, potting, composites, laminates, encapsulants for semiconductors, and insulating materials for electric devices, etc. o-Cresol formaldehyde novolac epoxy (CNE) is the resin typically employed in the encapsulation of microelectronic devices. Several approaches for modification of epoxy backbone for enhancing the thermal properties of epoxy resins have been reported. Aromatic bromine compounds in conjunction with antimony oxide are widely used as a flame retardant for epoxy resins. Tetrabromobisphenol A is a typical example of the aromatic bromine compounds used as a flame retardant for epoxy resins. An excess amount of epoxy resin is reacted with tetrabromobisphenol A to prepare an advanced epoxy resin having two terminal epoxide groups, as shown in the following formula: ##STR2##
A flame retardant advanced epoxy resin wherein EP is a bi-radical group of the backbone of the epoxy resin, and m is an integer of 1-10. The advanced epoxy resin can be used in preparing a flame-retardant printed circuit board (FR-4) by impregnating glass fibers with the advanced epoxy resin and heating the resulting composite to cure the advanced epoxy resin. Furthermore, the advanced epoxy resin can be employed to encapsulate microelectronic devices, in which the advanced epoxy resin is cured at a high temperature with a curing agent, so that an encapsulant having a flame-retardant property is formed. Typical examples can be found in U.S. Pat. No. 3,040,495 (1961); U.S. Pat. No. 3,058,946 (1962); U.S. Pat. No. 3,294,742 (1966); U.S. Pat. No. 3,929,908 (1975); U.S. Pat. No. 3,956,403 (1976); U.S. Pat. No. 3,974,235 (1976); U.S. Pat. No. 3,989,531 (1976); U.S. Pat. No. 4,058,507 (1997); U.S. Pat. No. 4,104,257 (1978); U.S. Pat. No. 4,170,711 (1979); and U.S. Pat. No. 4,647,648(1987)].
Although the tetrabromobisphenol A-containing advanced epoxy resin shows flame retardant property, major problems encountered with this system are concerned with the generation of toxic and corrosive fumes during combustion such as dioxin and benzofuran.
The flame retardant having a small molecular weight tends to degrade the mechanical properties of the epoxy resins, and migrate/vaporize from the epoxy resins such that the flame retardancy thereof diminishes.
Owing to organic phosphorus compounds generate less toxic gas and smoke than halogen-containing compounds, some authors have reported advanced epoxy resins containing phosphorus compound [Japanese patent application publication No. 10-30017 (1998), Japanese patent application publication No. 10-30016 (1998), Japanese patent application publication No. 10-152545 (1998)]. One example of the reaction is shown in the following scheme [Japanese patent application publication No. 10-30017 (1998)]: ##STR3##
Although these phosphorus containing advancement epoxy resins exhibited good flame retardancy, they were all derived from the reaction between aromatic phenol and epoxy group. For a multifuntional epoxy resin (functionality&gt;2), this advancement reaction may lead to gel if the reaction is not controlled well. These advancement epoxy resins yield low Tg product because they are derived from difunctional DGEBA (diglycidyl ether bisphenol A epoxy resin) and also due to their high EEW (epoxide equivalent weight) (EEW&gt;400 g/eq). In order to increase their Tg (glass transition temperature), multifunctional epoxy resin has to be added into these advanced resins. The blending of a multifunctional epoxy into these advanced resins may result in phase separation due to the difference in the reactivity between the multifuntional epoxy resin and the advanced epoxy resin toward the curing agent.
The trend of electronics equipment is being miniaturized and becoming thinner, at the same time the scale of integration of large scale integrated circuits (LSICs) is continuing upward, forcing the design toward larger chips, finer patterns, and higher pin counts that are more susceptible to a high-temperature failure. The prevailing surface mount technology (SMT) also causes the devices being subjected to a high temperature. Therefore, the development of a high-temperature reliable, flame-retardant and environmentally friendly epoxy resin for printed circuit board and encapsulant are essential for semiconductor industry.
It is an object of this invention to provide flame retardant advanced epoxy resins and cured epoxy resins with good thermal stability, superior heat resistance, and environment friendly, which are suitable for use in making printed circuit boards and in semiconductor encapsulation applications.
It is another object of this invention to provide a method for improving flame retardant properties of epoxy resins.